Genomic Background and Generation Time Influence Deleterious Mutation Rates in Daphnia

Abstract

Understanding how genetic variation is generated and how selection shapes mutation rates over evolutionary time requires knowledge of the factors influencing mutation and its effects on quantitative traits. We explore the impact of two factors, genomic background and generation time, on deleterious mutation in Daphnia pulicaria, a cyclically parthenogenic aquatic microcrustacean, using parallel mutation-accumulation experiments. The deleterious mutational properties of life-history characters for individuals from two different populations, and for individuals maintained at two different generation times, were quantified and compared. Mutational properties varied between populations, especially for clutch size, suggesting that genomic background influences mutational properties for some characters. Generation time was found to have a greater effect on mutational properties, with higher per-generation deleterious mutation rates in lines with longer generation times. These results suggest that differences in genetic architecture among populations and species may be explained in part by demographic features that significantly influence generation time and therefore the rate of mutation.A S the ultimate source of all genetic variation, mutation is an important evolutionary force affecting the ability of natural populations to respond to selective pressures. Most spontaneous mutations are deleterious (Lynch et al. 1999;Eyre-Walker and Keightley 2007), which is thought to explain many evolutionary phenomena, including inbreeding depression, mating system evolution, senescence, and risk of extinction to small populations Lynch et al. 1999). Despite the importance of knowing mutation rates in both theoretical and applied biology, few empirical estimates exist other than those for classic genetic model organisms (Baer et al. 2007), and little is known about the factors influencing the rate of mutation among individuals, populations, and species (Lynch 2010).In addition to direct estimates based on sequencing, estimates of the parameters for mutations affecting fitness [i.e., the genome-wide deleterious mutation rate (U) and the average effect (š)] have now been reported for several species (reviewed in Baer et al. 2007). However, little empirical attention has been given to variability in the phenotypic effects of deleterious mutation [i.e., per-generation rates of change in the mean phenotype (DM) and mutational variance (DV)] or to the associated deleterious mutation parameters that can be inferred from these quantities (U andš) among populations within a species. Recent theoretical treatments of mutation-rate evolution, however, predict individual variation in mutation rates (Lynch 2008;Desai and Fisher 2011) and fitness dependence of mutation rates (Agrawal 2002;Shaw and Baer 2011), highlighting the importance of this variability.The deepest understanding comes from recent mutationaccumulation studies in Drosophila melanogaster that provide evidence for variability in mutation rates among genotypes, using bot...

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